Manufacture of sodium



April 26, 1966 J. M. WOOD MANUFACTURE OF SODIUM Filed March 29, 1962FIG.

FIG. 2

United States Patent 3,248,311 MANUFACTURE OF SODIUM James M. Wood,Baton Rouge, La., assignor to Ethyl Corporation, New York, N .Y., acorporation of Virginia Filed Mar. 29, 1962, Ser. No. 183,444 2 Claims.(ill. 204-68) This invention relates to the art of producing metallicsodium and in particular to improvements in electrolytic cells,especially incells of the Downs type.

Metallic sodium is produced generally from amolten mixture of thechlorides of calcium and sodium in electrolytic cells of the Downs type(U.S. 1,501,756) or in modifications of these cells. These cells arecharacterized by having one or more bottom mounted vertically alignedcylindrical graphite anodes each of which is projected upwardly into aseparate cylindrical opening within a unitary cathode assembly. Thediameters of the anodes are smaller than the diameters of thecylindrical cathode openings so that the'combination of the surfaces ofthe graphite anodes and those of the cathode cylindrical openingsproduce annular openings which serve as electrolysis zones. Within eachof these zones is provided a foraminous metal diaphragm locatedgenerally equidistant from the anode surface and the cathode surface andits purpose is to effectively separate the anode and cathode surfacesinto compartments through which chlorine and metallic sodium areremoved. The sodium is produced at the cathode surfaces and the chlorineat the anode surfaces and because of the difference in densities betweenthese products, and that of the molten bath the products rise to thesurface of the bath and are then collected.

To collect the products, chlorine and sodium, a collector is mountedabove, though partially submerged within, the surface of the moltenbath. The collector is provided with an outlet and outlet port above theanodes for removal of the rising chlorine gas and also with a manifoldabove the cathode surfaces for collection of the sodium, which isgradually removed to the cell exterior.

Unfortunately, cells of this type rarely ever achieve currentefficiencies greater than about 83-86 percent and tremendous effort hasbeen exerted over past years to achieve greater current efliciency. Thereasons for these low efiiciencies however are not altogether known.Though not desiring to be bound by theory, the present invention isdirected toward the virtual elimination of certain types of parasiticcurrents which are believed by the instant inventor to exist. His theoryon the origination of certain types of parasitic currents is as follows.

The cell products, sodium and chlorine, though substantially physicallyseparated are nevertheless in electrical contact both through the metalwalls of the collector and through the cell bath. Thus, the metal wallsfrom an electronic conductor and the cell b ath provide an ionicconductor. This arrangement constitutes at least in effect a batterywhich consumes sodium from the collector by anodic oxidation andsimultaneously reduces chlorine on the opposite side of the wall of thecollector, and also on the anode side of the diaphragms. But, moreover,the driving force for the reionization of sodium and chlorine is notonly due to the nature of the chemical products and their electricalproximity but is even intensified by the electric field which existsbetween the anode and cathode itself. Thus, an additional driving forceis provided by the series of equipotential lines which extend outwardlyin a non-parallel direction from the anode and cathode. The net effectof these forces is that a certain amount of the electrical energy whichis externally applied to the electrodes for decomposition of the sodiumchloride is completely wasted. This type of parasitic current producedby this chemical battery arrangement is most unde- 3,248,311 PatentedApr. 26, 1966 ice sirable in that it lessens current efficiency,decreases production, and generally reduces cell performance.

Now, the direct object of this invention is to overcome the above andother disadvantages particularly as regards these types of parasiticcurrents within the present commercial process, or present commercialcell, and to provide for the more eflicient production of metallicsodium. Also, it is an object to provide a method for the reduction ofgalvanic corrosion of the metal parts of an electrolytic cell whichseparate sodium and chlorine. Further, it is an object to provide methodand apparatus which will lengthen diaphragm life, improve cellperformance by, inter alia, increasing production, by decreasingundesirable heating at the anode-cathode gap, and by lessening of thecorrosion of metal parts. Yet further, it is an object to provide a newand improved diaphragm structure.

These and other objects are achieved in accordance with the presentinvention which advances the art by providing for the reduction ofparasitic galvanic action by the use in cells of protective coatingsapplied upon metal surfaces or barriers which separate sodium fromchlorine.

Pursuant to this invention, it has been found that parasitic currents ofthe type described, generally reducing current efficiency by about 2percent or more, can be greatly reduced even as much as 99 percent bycoating the metal surfaces separating the sodium and chlorine with arefractory insulating material. Materials specially suitable for thispurpose are those refractory materials having a melting point aboveabout 700 C. and properly classi- =fied as electrical insulators.

The most preferred refractory insulating materials for the practice ofthis invention are those refractory oxides (ceramics) such as berylliumoxide, aluminum oxide, magnesium oxide, zirconium oxide, mixtures ofthese oxides, and also such oxide combinations including in additionsilicates and aluminates.

A particularly outstanding oxide, because of its very high chemicalresistance to attack by sodium because of its high shock resistance andits high melting point, inter alia, is aluminum oxide.

A major portion of the current efiiciency loss due to parasitic currentsresults from the parasitic currents at the diaphragms themselves andtherefore an especially preferred embodiment of this invention relatesto an improved foraminous metal diaphragm for location within an annularelectrolysis zone. The diaphragm comprises a cylindrical iron or steelwire structure, the individual gauze wires of which are impregnated orcoated with a refractory electrically insulating material of thecharacter described. The most particularly preferred coating however isalso a refractory electrically insulating material selected from thegroup of compounds consisting of oxides of aluminum, beryllium,magnesium, zirconium, and mixtures thereof and also such compounds whichinclude in addition aluminates and silicates.

These coatings when applied to diaphragm surfaces are particularlyeffective not only in reducing parasitic currents but also in reducingthe current density of these members so that less metallic sodium isactually formed on the anode side of the diaphragm and less chlorine isformed on the cathode side of the diaphragm. This additional desirableeffect is also of major importance and contributes to further increasethe overall current efficiency of the electrolytic cell within whichthese coated diaphragm members are used.

The thickness of the refractory-insulating materials is preferably fromabout 0.005 to about 0.1 inch and even more preferably from about 0.01to about 0.07 inch. A thickness of from about 0.01 to about 0.03 inch isfound to be particularly useful.

The nature of the invention and.the problems solved by its use will bereadily understood and appreciated by the following detailed descriptiongiven with reference to the figures wherein FIGURE 1 is across-sectional elevation view of a sodium cell of the Downs type.

FIGURE 2 is a foraminous metal diaphragm of the type used in such cells.

Referring to FIGURE 1 is shown the principal parts of a cellinstallation. Thus, in this figure is shown a generally cylindricallyshaped iron cell shell 10, the inside of which is lined with aninsulating material 11. Within the confines of the shell is mounted aunitary cathode assembly 20. Extended upwardly from the floor of thecell is a graphite anode 30. The peripheral surface of the anode 30 isconcentrically aligned with respect to the inner peripheral surface ofthe cathode assembly 20. Thus, the outer anode surface and the innersurface of the cathode assembly form the annular opening or electrolysiszone 21.

The unitary cathode assembly 20 is mounted within the cell shell 10 byprojections 22 22 which are bus bars electrically insulated, through theinsulator blocks 17, from the cell shell 10. An external current isapplied to the cathode through these bus bars 22 22 and current is alsoapplied to the anode 30 through bus bar 31.

Above the anode 30 and cathode assembly 20 is mounted acollector-diaphragm assembly, a portion of which assembly is submergedwithin the cell bath 46. This assembly is constituted by attachment ofdiaphragm 50 (shown in FIGURE 1 'by dotted lines) upon ports (detailsnot shown) located upon the lower portion of an inverted dome-shapedcollector 40. Within the collector is contained an inverted trough ormanifold 41 within which sodium is collected for transmission throughthe vaned riser pipe 42 to the cell exterior. Within the dome portion 43of the collector is passed chlorine which is sent via line 44 to thecell exterior for collection.

The foraminous diaphragm 50 is shown in more detail in FIGURE 2. Thediaphragm consists generally of a ferrous metal gauze which is stapledor otherwise fastened together in cylindrical shape. Upon the top of thediaphragm is secured a metal hoop 51 by means of which the diaphragm canbe attached to the collector 40.

To initiate operation, a mixture of salts including sodium chloride ischarged into the electrolysis cell. Sodium chloride is, of course, anessential part of this mixture and a second salt, or salts, is added forlowering of the melting point. In present commercial practice, a mixtureof calcium chloride and sodium chloride is used. Such mixtures arecharged into the cell and electrolysis of this mixture initiated.Operating temperatures range generally from about 525 C. to about 700C.; temperatures of from about 525 to about 625 C. being preferred.

As electrolysis proceeds, sodium is deposited upon the cathode surfaceand chlorine is deposited upon the anode surface of the electrolysiszone 21. Ideally, the deposited sodium will remain on the cathode sideof the diaphragm 50 and will rise through the molten bath to becollected within manifold 41. On the other hand, chlorine will remain onthe anode side of the diaphragm 50 of zone 21 and will rise through thebath to the dome 43. Even in this ideal state however, according to theinventors theory, parasitic currents are produced because of themetallic barriers which separate sodium and chlorine. Thus, the sodiummetal within the manifold 41 and the chlorine within the dome 43 areseparated by the metal walls 45 of the collector 40. Moreover, themetallic diaphragm gauze 50 located within the annular electrolysiszones 21 separates sodium from chlorine. These metallic separators,though substantially physically separating the sodium and chlorine, arenevertheless in electrical connection and give rise to parasiticcurrents. It is this parasitic galvanic action which the presentinvention almost substantially eliminates.

The following non-limiting examples are illustrative of the presentinvention.

Examples A series of l0 runs are made wherein both sides of ferrousmetal diaphragms are coated with aluminum oxide of a thickness of 0.03inch. These diaphragms in groups of 4 are simultaneously attached to acollector and inserted within an electrolysis cell of the type describedexcept that 4 anodes are mounted within the cell to form 4 separateelectrolysis zones. The electrolysis cell is operated at a temperatureof 575 C. and contains a fused melt consisting essentially of 64 Weightpercent calcium chloride and 36 weight percent sodium chloride. After 30days of continuous operation, it is found that the average currentefliciency is improved by 1.96 percent as contrasted with similar runswherein the diaphragms were uncoated.

Current efi iciency is further improved when the chlorine side of thecollector wall separating sodium and chlorine is spray coated with thealuminum oxide, a coat thickness of 0.07 inch being provided.

When the foregoing is repeated except that beryllium oxide, magnesiumoxide and Zirconium oxide are used in lieu of aluminum oxide, currentefficiency is also-significantly improved.

Barium aluminate, calcium zirconate and potassium aluminum silicate arealso found to improve current efficiency when these materials are spraycoated upon the metallic diaphragms and collector surfaces inthicknesses of 0.01, 0.05 and 0.1 inch, respectively.

It is apparent that the present invention is susceptible to considerablevariations without departing from its spirit and scope. Thus, inessence, the invention relates to the combinations of using anelectrically insulating refractory material as a coating for metallicsurfaces to separate sodium and chlorine; such combinations reducingparasitic galvanic action. These combinations are applicable to any andall types of cells wherein sodium and chlorine are produced and whereinthe sodium and chlorine are in electrical proximity.

As stated, the coating materials for the metal surfaces must be aninsulator in the traditional sense and must be capable of withstandinghigh temperatures. Such materials are those having a conductancegenerally on the order of 10 ohmcm.- or less. The melting point of thematerial must be as high as about 700 C. but should preferably begreater, preferably on the order of 1000 C. and above. A most preferredand highly unique class of materials for this purpose are thoserefractory oxides and ceramics such as the oxides of beryllium,aluminum, magnesium, zirconium, and mixtures of these compounds and alsoincluding oxide combinations of these materials wherein silicates andaluminates are also present.

Other refractory insulating materials include, for example, thesilicates and aluminates of alkali and alkaline earth metals to whichhave been added refractory oxides. Exemplary of such compounds arebarium aluminate, strontium zirconate, magnesium zirconate, magnesiumsilicate, magnesium zirconium silicate and sodium aluminum silicate.Other simple and complex refractory materials are also suitable for thepractice of this invention, especially those having melting points aboveabout 700 C. which are properly classifiable as electrical insulatorsand are suificiently resistant to chemical attack from sodium andchlorine.

These coatings can be applied to metals in various ways though a highlypreferred method is by spray coating. Pursuant to this method, therefractory oxide, a particularly unique material in this regard, isheated above its melting point, is volatilized, and then sprayed from asuitable container upon the metal surface to be coated. Upon coming incontact with this surface, the volatilized oxide condenses and adheresto the metal surface. Such refractory materials can also be applied byforming oxide slurries and dipping the metal to be coated therein. Uponbaking, a suitably shock resistant coating is adhered thereto.

The preferred thickness of the coatings ranges from about 0.005 to about0.1 inch but thinner or thicker coatings can be employed to provide someprotection. Thus, a coating thinner than 0.005 inch can be appliedthough this is generally undesirable because of chemical attack,especially by metallic sodium. Thicker coats are generally undesirablealso because they are less adherent than the thinner coats and leavesome surfaces exposed and also shock resistance is decreased to someextent.

Thicker coats are also generally undesirable. For one reason, thethickness of the coating is also somewhat dependent on the mesh size ofthe wire gauze Where a diaphragm is to be coated. Thus, the thickness ofthe coating must be compensated for by the size of the openings desired.Thus, a suitable gauze size is generally from about 20 to about 30(American standard) and if the coating becomes too thick, then obviouslythe gauze openings must be increased in area to compensate for thisthickness.

Other obvious modifications can also be made.

Having described the invention, what is claimed is:

1. In a process for the preparation of sodium by the electrolyticdecomposition of a fused sodium chloride bath conducted in a cellcomprising anode means and cathode means positioned relative to eachother to form an electrolysis zone wherein said means are separated by aforaminous diaphragm member to form an anode compartment in whichchlorine is liberated and a cathode com- 6 partment in which sodium isliberated, the improvement comprising employing as said member a ferrousmetal diaphragm member having an electrically insulating refractorycoating covering the entire surface area of said diaphragm exposed tosaid fused bath whereby parastic electric currents are essentiallyeliminated.

2. The process of claim 1 further characterized by said electricallyinsulating coating being aluminum oxide.

References (Iited by the Examiner UNITED STATES PATENTS 1,907,984 5/1933Kraner 204-181 2,150,289 3/1939 Moltkehansen 204-68 2,928,783 3/1960Bacon 204129 2,965,552 12/1960 Gruber 204- 3,011,964 12/1961 Guillot204295 3,022,244 2/1962 Le Blane et a1 204-295 3,098,802 7/1963 Beer204295 3,102,085 8/1963 Edwards et al. 204295 FOREIGN PATENTS 305,022 4/1930 Great Britain.

JOHN H. MACK, Primary Examiner.

JOHN R. SPECK, WINSTON A. DOUGLAS,

Examiners.

B. JOHNSON, H. S. WILLIAMS, Assistant Examiners.

1. IN A PROCESS FOR THE PREPARATION OF SODIUM BY THE ELECTROLYTICDECOMPOSITION OF A FUSED SODIUM CHLORIDE BATH CONDUCTED IN A CELLCOMPRISING ANODE MEANS AND CATHODE MEANS POSITIONED RELATIVE TO EACHOTHER TO FORM AN ELECTROLYSIS ZONE WHEREIN SAID MEANS ARE SEPARATED BY AFORAMINOUS DIAPHRAGM MEMBER TO FORM AN ANODE COMPARTMENT IN WHICHCHLORINE IS LIBERATED AND A CATHODE COMPARTMENT IN WHICH SODIUM ISLIBERATED, THE IMPROVEMENT COMPRISING EMPLOYING AS SAID MEMBR A FERROUSMETAL DIAPHRAGM MEMBER HAVING AN ELECTRICALLY INSULATING REFRACTORYCOATING COVERING THE ENTIRE SURFACE AREA OF SAID DIAPHRAGM EXPOSED TOSAID FUSED BATH WHEREBY PARASTIC ELECTRIC CURRENTS ARE ESSENTIALLYELIMINATED.